The shooting occurred at the Erez checkpoint, the main entrance used by relief agencies to funnel badly needed food and medical supplies into Gaza, where Israel is waging a devastating, 13-day-long military campaign against the militant Islamic group Hamas.

U.N. officials said that they had contracted the truck to deliver supplies into Gaza, and that the Israeli military had approved the delivery. But Israeli ground troops, which control the Erez checkpoint, fired on the truck. It wasn’t clear what caused the Israeli soldiers to open fire, and Israeli military officials weren't immediately available for comment.

Relief officials said that as a result of the shooting, Israel closed the Erez checkpoint. It was unclear whether any relief trucks would be allowed to enter Gaza on Thursday, despite what humanitarian agencies describe as a worsening humanitarian crisis.

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"It's a tragic example of how impossible it’s becoming to deliver assistance here," John Ging, the top U.N. refugee official in Gaza, told the Al Jazeera television network. "We cannot continue in these circumstances where aid workersare being killed and injured, even when they are in direct coordination with the Israeli liaison people, who are supposed to ensure their safety."

It was at least the second time in recent days that Palestinian relief workers contracted by the U.N. had been killed. On Monday, Israeli shells landed near the parking lot of a Palestinian company contracted by the U.N. World Food Program to deliver food supplies in Gaza. Two employees were killed and three severely wounded, according to WFP spokeswomen Barbara Conte, who said that the relief workers were not the target of the Israeli strike.

Nearly 700 Palestinians have died in Gaza since the conflict began, including a rapidly rising number of women and children, and nearly 3,000 have been wounded, according to Gaza health officials.

Israel planned to observe another three-hour pause in fighting Thursday afternoon to allow besieged Gaza residents — most of whom are running out of food and have had their water and electricity sources cut off due to the fighting — to emerge from their homes and collect food, cooking gas, blankets and other supplies.

But in the hours before the pause, Israeli forces continued to pound the narrow coastal territory, bombarding approximately 60 targets overnight including the homes of two leading Hamas militants.

An Israeli soldier was killed and another lightly wounded when militants fired on their brigade with an anti-tank missile during a firefight Thursday morning in central Gaza. The death marked the seventh military fatality since Israel sent ground forces into Gaza five days ago.

Also Thursday, a burst of rockets from southern Lebanon landed in northern Israel, but the Israeli military said it did not consider the attack a sign that the war in Gaza could expand.

At least three rockets landed near the Israeli town of Nahariya — one of them lightly injuring two women in a nursing home — and the Israeli military fired a volley of artillery shells back across the border in response.

The Israeli military downplayed the incident, calling it "an isolated event."

"We don’t expect there to be more" rocket fire from Lebanon, Major Avital Leibovich said.

No Lebanese group immediately claimed responsibility for the attack. Hezbollah, which fought a war with Israel in 2006 during which it lobbed more than 3,000 Russian-made Katyusha rockets into northern Israel, has condemned the war on Hamas but so far stayed on the sidelines of the conflict. Smaller pro-Palestinian militant groups in Lebanon also possess rockets.

Special correspondent Ahmed Abu Hamda contributed to this report from Gaza City.